The present invention relates, in general, to casting devices for making investment molds and, more particularly, to a new and useful ringless casting oval for making investment molds for precision casting of dental prosthesis.
Investment casting, also known as precision casting, or the lost wax technique, is used in dental practice to create a casting from a wax pattern that is a duplicate of the object to be cast. The wax pattern is an acurate replica of the shape of a metal or glass dental restoration such as a gold crown, partial denture framework, metal coping for a porcelain-fused-to metal crown or a cast glass crown (e.g. the Ivoclar-Empress System). The wax pattern is attached to a second piece of wax or plastic known as a sprue.
The sprue is attached to a conical sprue former in a base to hold the wax pattern in place. A casing is then placed around the wax pattern and attached to the base to form a vessel or container into which an investment solution is poured. The investment material then hardens around the wax patterns to form the "investment mold". Removal of the base leaves a conical shape in the investment mold which will direct molten metal into the investment mold. The wax, and, if present, plastic is first removed from the mold space by placing the investment mold into an oven to melt and burn out the wax and plastic.
In many known casting mold devices, a metal or plastic casing is force fit into a groove in the base to form the vessel for holding or receiving the investment solution.
The casing may be a circular ring which fits into a circular base or the casing may be an oblong so-called oval-shaped oval which fits into an oblong or oval-shaped base to form the vessel.
In systems known as "ringless" systems, the casing is separated from the investment mold prior to the wax burnout. Ringless casting procedures eliminate many time-consuming steps associated with conventional casting procedures, which are metal casings, such as applying internal ring liners, digging burned out molds with casting out of metal rings and cleaning baked-on investment from the metal rings. The use of plastic casings from ringless molds is particularly suited to quick and easy divesting after casting and to easy cleanup. Durable plastic rings may be reused many times thereby reducing the cost per use. While techniques for oval and round casings are similar, the use of oval-shaped casings provides certain benefits in casting because investment material is used more conservatively and because the investment solution surrounds the wax pattern more uniformly which helps ensure an evenly heated investment mold and reduces the possibility of cold spots which can cause incomplete castings.
The use of flexible polymer such as polyvinyl chloride in the construction of both the base and ring is desirable, among other reasons, as such materials are more readily cleaned after the hardened investment mold is removed from the vessel.
A ringless casting oval and base device, as illustrated in U.S. Pat. No. Des. 329,900, is marketed by Belle de St. Claire of Chatsworth, Calif. The device comprises an oblong or oval-shaped casing and base, forming a vessel which receives investment solution.
In use, a wax form is mounted on the base. The casing is then inserted on or over the base. A projection provided on the inner surface of an upstanding flange engages a complementary groove formed in the outside of the casing to provide a mechanical indication that the casing is properly seated in the base.
Both the oval casing and base are composed of a flexible polyvinyl chloride. Since the polyvinyl chloride is not rigid but flexible, it may compress or distort when handled or as a result of the setting expansion, hygroscopic expansion or thermal expansion of the investment.
The Belle de St. Claire design provides a dental casting vessel which efficiently seals the investment solution within the investment vessel. However, since the casing and base are made of a flexible plastic, there may be an undesired, limited relative rotation between these parts.
Accordingly, there remains a need for improvements that includes the benefits of using a ringless casting oval without exhibiting the limited rotational tendency between the casing and base.